As an iconic unit that can rally people around its banner in dark times, there can be few more regiments more worthy than this one.

" The Marik Guard has been a part of the armed forces of the Marik Commonwealth since the Commonwealth was first founded. "

They were rebuilt, just not as the Marik Guards. FM:FWL p35-36 and p80 explain the situation, which I've paraphrased below.

The Marik Guard were nigh on decimated on Callison during the Fourth Succession War by the Hsein Hotheads. Their surviviors went on to form the core of the First Free Worlds Legionnaires, along with survivors of the Defenders of Andurien following the Andurien War, and entered service in 3042.